How to fix SIP errors
Softphone.Pro is a SIP client. To make and receive calls, it must be configured with your SIP account details and connected to your PBX.
If something goes wrong, your PBX may return a SIP response code. These codes can be difficult to interpret, especially if you are new to VoIP. This guide explains what the most common SIP errors mean and how to resolve them.
401 Unauthorized
A 401 response is part of the standard SIP authentication process and may briefly appear during normal registration. However, if registration fails and the server keeps returning 401, the SIP credentials are usually incorrect.
Check the SIP server address, username, authentication ID, and password provided by your VoIP provider or PBX administrator. Make sure there are no typos, extra spaces, or outdated credentials.
A persistent 401 response may also occur if the same SIP account is registered on multiple devices and the server does not allow simultaneous registrations. If the credentials are correct but the problem continues, contact your PBX administrator or VoIP provider.
402 Payment Required
Some VoIP providers use a 402 response to indicate a billing or account-related problem. For example, your account balance may be too low, or a service required to place calls may be disabled.
This response is not related to your Softphone.Pro license. If the license has expired, the app displays a separate message: “The license key has expired.”
403 Forbidden
A 403 response means that the server understood the request but refused to process it. The exact reason depends on your PBX or VoIP provider.
Common causes include:
- Incorrect SIP credentials
- A disabled, blocked, or deleted SIP account
- Registration from multiple devices when simultaneous registrations are not allowed
- Registration from a restricted country, network, or IP address
- A VPN or proxy connection blocked by the PBX
- A restricted destination or an incorrect dialing prefix
Check your SIP credentials and account status, verify the dialing format, and temporarily disable your VPN or proxy. If the error persists, contact your PBX administrator or VoIP provider.
404 Not Found
A 404 response means that the server could not find the user or domain specified in the request. During a call, this usually indicates that the number, extension, SIP address, or domain is incorrect or not recognized by the server.
Check the dialed number and make sure it includes the correct country code, area code, and any prefix required by your PBX. Some systems require numbers without the plus sign or use a specific access prefix for outbound calls.
If the number is correct, the destination may not exist, or the required route may not be configured on the PBX. Contact your PBX administrator or VoIP provider for assistance.
407 Proxy Authentication Required
A 407 response is an authentication challenge from a SIP proxy. Like a 401 response, it may briefly appear during normal registration or call setup. The client then resends the request with the credentials configured for the SIP account.
If the proxy keeps returning 407 and the registration or call fails, check the SIP username, authentication ID, password, server address, port, and transport protocol. Make sure the credentials contain no typos or extra spaces.
If all settings are correct but the problem persists, contact your PBX administrator or VoIP provider. The proxy authentication method or server configuration may need to be checked.
408 Request Timeout
A 408 response means that a SIP server or proxy did not receive the expected response within the allowed time.
Possible causes include:
- An incorrect SIP server address, port, or transport protocol
- A temporary network or DNS problem
- A firewall, router, or security application blocking SIP traffic
- A PBX that is unavailable or accessible only through a VPN
- A timeout on the provider’s network or at the destination
First, check that your internet connection is working. Keep in mind that being able to open websites confirms only general internet access—it does not guarantee that the SIP server is reachable.
Next, verify the SIP server address, port, and transport protocol in your account settings. If possible, try another network or ask your network administrator to confirm that SIP traffic is allowed.
If the error occurs only when calling a particular number, the problem may be related to the provider’s route or the destination. If it affects registration or all calls, contact your PBX administrator or VoIP provider.
480 Temporarily Unavailable
A 480 response means that the destination is currently unavailable, although it may become reachable later.
The called device may be offline, unregistered, outside the network coverage area, or temporarily unable to accept calls. Some PBXs and VoIP providers may also return this response when no valid route or forwarding destination is available.
Try calling again later. If the error occurs only when calling a particular number or extension, the problem is most likely on the destination side. If it affects many or all calls, check that your SIP account is registered and contact your PBX administrator or VoIP provider.
482 Loop Detected
A 482 response means that a SIP server has detected a routing loop. Without stopping the request, it would continue circulating between the same servers or routes.
This is usually caused by an incorrect routing, forwarding, or DNS configuration on the PBX or provider side. It cannot normally be resolved in Softphone.Pro. Contact your PBX administrator or VoIP provider and provide the destination number and time of the failed call.
487 Request Terminated
A 487 response means that a pending request was canceled before it could be completed. For example, it may appear when you end an outgoing call before the other party answers.
It may also be generated when a PBX tries several destinations or devices and cancels the remaining attempts after one of them answers. In most cases, this is normal SIP behavior and requires no action.
If calls are being terminated unexpectedly, contact your PBX administrator or VoIP provider and provide the destination number and time of the failed call.
488 Not Acceptable Here
A 488 response means that the destination cannot accept the media parameters offered for the call. The request may still succeed when sent to another device or destination.
Common causes include:
- No compatible audio codec
- Unsupported media encryption
- Incompatible audio or video settings
- Incorrect SDP handling by the PBX or VoIP provider
Check which codecs and media settings are supported by your PBX or VoIP provider. If the provider does not support encrypted media, disable Media encryption in the SIP account settings and try the call again.
If the error persists, contact your PBX administrator or VoIP provider. The SIP logs and SDP offer may be required to identify the incompatible parameter.
500 Server Internal Error
A 500 response means that the SIP server encountered an unexpected condition and could not process the request. This is generally a server-side problem and cannot usually be resolved by changing the settings in Softphone.Pro.
The problem may be temporary, so wait a few seconds and try again. If the response includes a Retry-After value, wait for the specified period before retrying.
If the error persists, contact your PBX administrator or VoIP provider. Provide the destination number, the time of the failed call, and information about whether the error affects registration, all calls, or only specific destinations.
502 Bad Gateway
A 502 response means that a SIP server acting as a gateway or proxy received an invalid response from a downstream server while processing the request.
The problem is usually located somewhere between your PBX and another server involved in routing the call. Try again later. If the error occurs only when calling particular numbers, it may be related to a specific route or destination.
If the error persists or affects all calls, contact your PBX administrator or VoIP provider and provide the destination number and time of the failed call.
503 Service Unavailable
A 503 response means that a SIP server is temporarily unable to process the request. The server may be overloaded, undergoing maintenance, or experiencing another temporary problem.
Wait a short while and try again. If the response includes a Retry-After value, wait for the specified period before retrying.
The response may come from your PBX, VoIP provider, or another server involved in routing the call. If the error occurs only occasionally, it is likely to be a temporary server-side problem. If it persists or affects all calls, contact your PBX administrator or VoIP provider and provide the destination number and time of the failed call.
603 Decline
A 603 response means that the call was explicitly declined by the destination or by a server acting on its behalf.
The called party may have rejected the call, or a PBX or VoIP provider may have blocked it according to its routing or access rules. Some providers may also use this response for account-related restrictions, although this is provider-specific.
Try another number or contact the called party through another channel. If calls to multiple destinations return the same response, check your account status and contact your PBX administrator or VoIP provider.